1. Field of the Invention
This invention relates to microwave transmission circuits. More particularly, the invention relates to a transition circuit for joining a microstrip with an inverted microstrip.
2. Description of the Prior Art
Those concerned with the development of planar microwave devices having large sections of microstrip have recognized that in some instances their performance could be enhanced significantly if inverted microstrip could be substituted for portions of the microstrip. It is known that at microwave frequencies higher than 26 GHz, for example, significant losses commonly occur in the dielectric substrates of conventional microstrip. In many situations these losses may be reduced substantially with the use of inverted microstrip because most of the electromagetic energy in inverted microstrip is transmitted in the low-loss air space between the strip conductor and the ground plane.
Also, in many planar circuit designs it is physically more convenient and cheaper to use inverted microstrip instead of microstrip. This is particularly the case in the fabrication of thin-film superconducting planar microwave devices. Conventional superconducting films are currently deposited on only one side of a dielectric substrate, making them difficult to use in a microstrip configuration where both sides of a dielectric substrate are coated with a conductive film.
Although it has been recognized that in many situations it is desirable to use both microstrip and inverted microstrip in a common circuit, no practical apparatus for making low-loss connections between microstrip and inverted microstrip has yet been devised. Ideally, such apparatus should provide a low-cost, low-loss transition between microstrip and inverted microstrip. The present invention fulfills this need.